The Performance of the Neck Circumference for a Difficult Laryngoscopy in Obese Patients

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1 The Performance of the Neck Circumference for a Difficult Laryngoscopy in Obese Patients Arunotai Siriussawakul MD*, Sirirat Rattana-arpa MD*, Sukanya Jirachaipitak MD*, Anan Chatsiriphattana MD*, Akarin Nimmannit MD**, Nujira Wong-in BNS* * Department of Anesthesiology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand ** Office for Research and Development, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand Background and Objective: There is conflicting evidence as to whether neck circumference (NC) is related to a difficult laryngoscopy in obese patients. The objectives of this study were to determine the performance of the NC in defining a difficult laryngoscopy, and to identify factors predicting a difficult laryngoscopy among those obese patients. Material and Method: This prospective study was conducted in adult patients with a body mass index (BMI) of greater or equal to 30 kg/m 2 who underwent conventional endotracheal intubation. Preoperative NC, the modified Mallampati test (MMT), the thyromental distance (TM) and the sternomental distance (SM) were measured. A difficult laryngoscopy was defined as a Cormack-Lehane laryngoscopic view of grade III or IV. Results: Five hundred patients (366 females; 134 males), with a mean BMI of kg/m 2 (minimum = 30.0 kg/m 2, and maximum = 68.4 kg/m 2 ) and who had elective surgery planned, were enrolled. The incidence of a difficult laryngoscopy was 7.2%. The overall performance of the NC using the area under the receiver operating characteristic curves of a difficult laryngoscopy was 0.574, with a 95% confidence interval (CI) of 0.53 to The optimal cutoff point of the NC for a difficult laryngoscopy was 37.1 centimeters. It provided sensitivity and specificity of 75.5% (95% CI 57.8, 87.9) and 42.5% (95% CI 37.9, 47.2). The positive and negative predictive values were 9.2% and 95.6%. After multivariate analysis, factors affecting a difficult laryngoscopy were high grades of MMT (odds ratio (OR) 2.23, 95% CI 1.10, 4.50) and NC/TM (OR 1.78, 95% CI 1.15, 2.74). Conclusion: The NC per se was not a good predictor of a difficult laryngoscopy. However, the ratio of the NC to the TM and the MMT were factors predicting a difficult laryngoscopy in obese patients. Keywords: Obesity, Difficult intubation, Clinical prediction, Neck circumference J Med Assoc Thai 2016; 99 (5): Full text. e-journal: Obesity has long been alleged to be a risk factor for difficult airway management even though most obese patients do not have apparently abnormal airway characteristics. The increased amount of fat deposition anterior to the laryngopharyngeal air space may impair the mobility of the pharyngeal structure, and the fat could, therefore, cause a collapse of the airway during general anesthesia (1). Both of these changes may be important risk factors for a difficult laryngoscopy in these obese patients. The quantification of neck tissue can be determined by imaging techniques, including X-rays, computed tomography (CT) scans and magnetic resonance Correspondence to: Siriussawakul A, Department of Anesthesiology, Faculty of Medicine, Siriraj Hospital, Mahidol University, 2 Wanglang Road, Bangkoknoi, Bangkok 10700, Thailand. Phone: , Fax: arunotai.sir@mahidol.ac.th imaging (MRI), or performing ultrasonography (2). However, all of those investigations are costly and need excessive time and expertise. Therefore, it is not practical to perform them in all pre-operative airway assessment in clinical practice. The neck circumference (NC) is a useful tool for predicting metabolic syndrome. Men with an NC of 39 centimeters (cm) or greater, and women with an NC of 33 cm or greater, are at risk of establishing hypertension, diabetes mellitus, dyslipidemia and central obesity (3). Apart from the routine bedside tests, which are the modified Mallampati test (MMT), the thyromental distance (TM) and the sternomental distance (SM), the NC has been proposed to be a new predictor of a difficult intubation in obese patients for a decade (4). The perceived advantage of employing the NC is that this method of measurement is easy and reliable. The test does not depend on patients cooperation, and the results do not differ when the 484 J Med Assoc Thai Vol. 99 No

2 patients feel exhausted after repeated measurements. The intraclass correlation coefficient between researchers is very high compared to other tests (5). Nevertheless, there have been conflicting results for the predictive ability of the NC for a difficult intubation in obese patients (6,7). Therefore, we conducted this study for two objectives: firstly, to determine the performance of the NC to define a difficult laryngoscopy, and secondly, to identify factors predicting a difficult laryngoscopy among those obese patients. Material and Method This study was part of a multi-center study. The presented study was approved by the Institutional Review Board (IRB), and informed consents were obtained from all participants. Inclusion criteria were obese adult patients aged 18 or more with a body mass index (BMI) of 30 kg/m 2 or greater, and scheduled to undergo general anesthesia with endotracheal intubation for elective surgery. If the patients had any one of the following exclusion criteria, they were excluded: patient refusal, patients with obvious malformations or pathology of the upper airway, cervical spine fracture, pregnancy, and history of difficult airway management (specifically, intubation by a special technique such as awake fiberoptic intubation or video laryngoscope). Airway assessment tests Pre-operative airway assessments, including the NC, the MMT, the TM and the SM, were evaluated by a group of well-trained research assistants on the day before surgery. The MMT was assessed while a patient sat upright with the head in a neutral position.the patient was asked to open his or her mouth as wide as possible, and to protrude the tongue to the maximum without phonation. The NC was measured at the level of the cricoid cartilage, perpendicular to the long axis of the neck. The TM and the SM were measured in the seated position, with the head fully extended and the mouth closed. The straight line between the thyroid notch and the bony point of the mentum (the TM), and the straight distance between the upper border of the manubrium sterni and the bony point of the mentum (the SM) were measured by using a ruler. Anesthetic protocol Standard monitors, including an electrocardiogram, a pulse oximetry and a non-invasive arterial blood pressure monitor, were utilized before the anesthetic induction period. Preoxygenation was conducted by administering 100% oxygen via a facemask for more than three minutes. The endotracheal intubations were performed by an experienced anesthetist who had worked in the operating theater for at least two years. An independent, in-charge anesthesiologist monitored patients status and made decisions during the intraoperative period, including the patients positions, the choice of anesthetics, the types of laryngoscope blades as well as the management of any unanticipated difficult-airway events. Definition of the outcome criteria A difficult laryngoscopy was defined as grade III or IV of the Cormack-Lehane laryngoscopic view. The Cormack-Lehane laryngoscopic view was classified into four grades as follows: grade I view: the vocal cords were completely visible; grade II: only the arytenoids were visible; grade III: only the epiglottis was visible; and grade IV: the epiglottis was not visible. The invisible laryngeal inlet might be associated with a difficult intubation (8). Statistical analysis The sample size calculation was based on an expected the NC s ability to classify correctly a patient as having a Cormack-Lehane laryngoscopic view either of grade III or IV, or of grade I or II. A prospective power analysis was performed to determine the number of cases with a Cormack-Lehane laryngoscopic view of grade III or IV that would be required. This showed that a sample size of 35 patients with a Cormack-Lehane laryngoscopic view of grade III or IV would yield an anticipated sensitivity and specificity of 90% with 10% range of 95% CI (two-tailed). The expected prevalence of difficult intubations in obese patients arose from previous literature, which was reported as 13.8% (9). Hence, the total number of obese subjects required was at least 250. The statistical analysis was conducted using a software program, SPSS version 18.0, SPSS Inc., Chicago, IL, USA. Descriptive statistics were used to examine the patients characteristics and airway assessment tests. A receiver-operating characteristic (ROC) curve was plotted to represent the overall accuracy and the optimal cutoff point of the NC for identifying of a difficult laryngoscopy. The performances of the NC test were summarized by the sensitivity, specificity, positive predictive value (PPV), J Med Assoc Thai Vol. 99 No

3 negative predictive value (NPV) and likelihood ratios. As for identifying factors predicting a difficult laryngoscopy, the BMI and four, airway-assessment tests were compared using Chi-square or Fisher s exact test for non-continuous variables, and two-tailed student s t test for continuous variables. Subsequently, variables with p<0.2, or clinical meaningful variables, were entered into a multiple logistic regression analysis of patients with a difficult laryngoscopy. The noncontinuous data were presented as number of occurrences and percentage, while the continuous data were presented as mean + standard deviation (SD). The crude odds ratio, the adjusted odds ratio and the 95% CI were reported to consider the strength of association between the types of airway assessment tests and the patients with a difficult laryngoscopy. The statistical significance was defined as a p-value of smaller than Results A prospective observational study was conducted at a tertiary university-based hospital, during the 18-month period from 1 March 2013 to 30 September Five hundred obese patients (366 females; 134 males), with a mean BMI of kg/m 2 (minimum = 30.0 kg/m 2 ; maximum = 68.4 kg/m 2 ) and for whom planned elective surgery was undertaken, were recruited to fulfill the study termination criterion of having at least 35 patients with a difficult laryngoscopy. Other demographic data and preoperative assessment tests of the patients are shown at Table 1. In the operating theatre, most patients were placed in a sniffing position (91%), and most direct laryngoscopies were successful performed by using a McIntosh laryngoscope blade number 3 (85.8%). General anesthesia was mostly induced by propofol (96.6%), and patients were intubated by administering non-depolarizing muscle relaxants (75.2%) or succinylcholine (24.8%). The distributions of the percentages of the Cormack-Lehane laryngoscopic views were 61.6%, 31.2%, 6.0% and 1.2% for views I, II, III and IV, respectively. Overall, the incidence of a difficult laryngoscopy then was equal to 7.2% (36 out of 500 patients). No failed intubation was reported in this study. The overall performance of the NC using the area under the receiver operating characteristic curves of a difficult laryngoscopy was 0.574, with a 95% confidence interval (CI) of 0.53 to (Fig. 1). The optimal cutoff point of the NC for a difficult Table 1. Demographic data and preoperative airway assessment tests Variables Mean + SD or number (%) Age (years) Gender Female 366 (73.2) Male 134 (26.8) ASA classification: II, III 435 (87.0), 65 (13.0) Department General surgery 106 (21.2) Head-neck-breast 96 (19.2) ENT, Eye 65 (13.0), 27 (5.4) Gynecology 64 (12.8) Orthopedics 63 (12.6) Neurology 33 (6.6) Others 46 (9.2) Comorbidities None 145 (29) Hypertension 229 (45.8) Diabetic mellitus 109 (21.8) Obstructive sleep apnea 21 (4.2) Body mass index (kg/m 2 ) Modified Mallampati test Class I, II 114 (22.8), 171 (34.2) Class III, IV 124 (24.8), 91 (18.2) Neck circumference (cm) Thyromental distance (cm) Sternomental distance (cm) ASA = The American society of anesthesiologists; ENT = ear, nose and throat; SD = standard deviation laryngoscopy was 37.1 cm. It provided sensitivity and specificity of 75.5% (95% CI 57.8, 87.9) and 42.5% (95% CI 37.9, 47.2). The positive and negative predictive values were 9.2% and 95.6% (Table 2). As for factors predicting a difficult laryngoscopy, five physical examinations were included in univariate analysis. In order to avoid the multicollinearity problem, only three independent factors were entered into a multiple logistic regression model (Table 3). Two factors were found to be related to a difficult laryngoscopy: high grades of MMT (odds ratio (OR) 2.23, 95% CI 1.10, 4.50) and the NC/TM (OR 1.78, 95% CI 1.15, 2.74). Discussion The scheme of glottic views developed by Cormack and Lehane (7) has become a standard measurement, and it facilitates communication between clinicians and researchers. A number of definitions of a difficult airway sometimes refer to a high-grade 486 J Med Assoc Thai Vol. 99 No

4 laryngeal view, even though it does not always lead to a difficult intubation. As shown in both a study by Arne et al and in our study, many of the patients with Cormack-Lehane grade III and IV views were not associated with the occurrence of a difficult or a failed intubation (10). Nevertheless, the possibility of there being a difficult laryngoscopy in obese patients should be considered. Comparing to a previous study conducted in Thailand, the incidence of a difficult laryngoscopy in obese patients was higher than in the general population (11). Failure to see the glottis by line of sight may result in troublesome management, the need to change airway equipment, or a long intubation Fig. 1 Receiving Operating Characteristic (ROC) curve of the neck circumference among obese patients. time (12). Since the patients have a number of physiologic alterations, such as decreased functional residual capacity, decreased chest wall compliance and increased oxygen consumption (13), the awkward situation may lead to adverse perioperative events. There is ongoing research to identify a single, routine, airway-assessment test to predict the occurrence of a difficult airway. Shiga et al reported common bedside tests for the prediction of a difficult intubation, including the Mallampati classification, the inter-incisor gap, the TM and the SM. However, each test yielded poor to moderate pooled sensitivity (14). The NC has been an attractive screening test for predicting a difficult laryngoscopy in obese patients as it is simple and reasonable. Gonzalez et al demonstrated that an NC of more than 43 cm was a good predictor for a difficult intubation, with a sensitivity of 92% (6). Brodsky et al also showed that the probability of a problematic intubation increased 5% if the NC was at 40 cm, and the probability was 35% when the NC was at 60 cm (4). Compared with our study, the optimal cutoff point of the NC for a difficult laryngoscopy was only 37.1 cm, with the area under the receiver operating characteristic curves of (95% CI 0.53, 0.618). The poor predictive value of the NC in our study could be explained by different ethnics. Europeans tend to have a larger body composition than Asians. Three independent factors were entered into a multiple logistic regression model. The variables were chosen based on previous reports and concerns about having a sufficient number of events per variable (15). Weight-for-height indices have long been used to indicate the amount of body fat accumulation. The relationship between different cutoff points of the BMI and the risk of comorbidities has been demonstrated in several studies (3,16). However, the BMI Table 2. The performance of the neck circumference based on ROC curve analysis NC (cm) sensitivity specificity PPV NPV LR+ LR cm = centimeters; PPV = positive predictive value; NPV = negative predictive value; LR+ = positive likelihood ratio; LR- = negative likelihood ratio J Med Assoc Thai Vol. 99 No

5 Table 3. Factors associated with difficult laryngoscopy Factors Mean + SD or number (%) Crude OR p-value 1 Adjusted OR p-value 2 (95% CI) (95% CI) LV I-II LV III-IV (n = 464) (n = 36) BMI (kg/m 2 ) < (62.8) 28 (5.6) > (30) 8 (1.6) 0.60 (0.27, 1.34) (0.24, 1.22) 0.14 MMT Class I, II 271 (54.2) 14 (2.8) Class III, IV 193 (38.6) 22 (4.4) 2.20 (1.10, 4.42) (1.10, 4.50) 0.03 NC (cm) TM (cm) SM (cm) NC/TM (1.16, 2.70) (1.15, 2.74) 0.01 BMI = body mass index; MMT = modified Mallampati test; NC = neck circumference; TM = thyromental distance; SM = Sternomental distance; NC/TM = the ratio of neck circumference to thyromental distance; LV = Cormack-Lehane laryngoscopic view; cm = centimeters; SD = standard deviation. p-value 1 came from univariate analysis, p-value 2 came from multiple logistic regression analysis cutoff point which refers to morbid obesity did not reflect fat distributions. We could not assume that a high BMI would correlate with an increase in adipose tissue deposits in the head and neck region. As a result, the BMI was not a good predictor of a difficult laryngoscopy in obese Thai patients. Two significant factors affecting a difficult laryngoscopy were high grades of MMT and the ratio of the NC to the TM. The MMT seemed to be useful in general patients as well as in obese patients (11). The insertion of the McIntosh laryngoscope blade requires either the compression or the retraction of the base of the tongue anteriorly to reveal the larynx. A high grade MMT may indicate that a large tongue, having occupied most of the area in the oral cavity, had obscured visualization of the laryngeal structure. Kim et al proposed a new index that assumed a difficult intubation would occur in obese patients who had a big or a short neck. The NC/TM might represent the distribution of fat in the neck better than the NC alone (9). According to the result of Kim s study, we may conclude that the NC/TM was a good predictor for predicting a difficult laryngoscopy and a difficult intubation among Asians. This study had some limitation. First, we could not establish what were the authentic causes of the difficult laryngoscopies among these obese patients. In addition, we did not collect other details of anthropometric measurement. Further studies should try to enroll patients who had a fat distribution predominately at their head and neck area instead of enrolling all obese patients diagnosed with a BMI above a high cutoff point. In conclusion, our results demonstrated that the NC did not properly predict a difficult laryngoscopy among obese patients. Instead, the difficulty was independently associated with high grades of MMT and large and short necks, which were represented by the ratio of the NC to the TM. What is already known on this topic? The neck circumference (NC) has been proposed to be a new predictor of a difficult intubation in obese Europeans. A problematic intubation increased if the neck circumference was at 40 cm, and the probability was 35% when the circumference was at 60 cm. What this study adds? The NC alone was not a good bedside test for predicting a difficult laryngoscopy, but the combination of NC and TM, and the MMT were factors predicting a difficult laryngoscopy among obese Thai patients. Acknowledgements The authors thank Miss Nichapat Sooksri, Miss Chusana Rungjindamai and Miss Kamonpan Homchuangsub, research assistants, for their 488 J Med Assoc Thai Vol. 99 No

6 invaluable help with the paperwork. Supported by Siriraj Research Development Fund [managed by Routine to Research (R2R)], Siriraj Hospital, Bangkok, Thailand. Potential conflicts of interest None. References 1. Ezri T, Gewurtz G, Sessler DI, Medalion B, Szmuk P, Hagberg C, et al. Prediction of difficult laryngoscopy in obese patients by ultrasound quantification of anterior neck soft tissue. Anaesthesia 2003; 58: Horner RL, Mohiaddin RH, Lowell DG, Shea SA, Burman ED, Longmore DB, et al. Sites and sizes of fat deposits around the pharynx in obese patients with obstructive sleep apnoea and weight matched controls. Eur Respir J 1989; 2: Limpawattana P, Manjavong M, Sopapong R. Can neck circumference predict metabolic syndrome? An experience from a university community. Endocr Pract 2016; 22: Brodsky JB, Lemmens HJ, Brock-Utne JG, Vierra M, Saidman LJ. Morbid obesity and tracheal intubation. Anesth Analg 2002; 94: Siriussawakul A, Wangdee A, Chatsiriphattana A, Komoltri C, Dejarkom S, Jirachaipitak S, et al. Modified Mallampati test, neck circumference and short inter-incisor gap are risk factors for troublesome intubation in Thai obese patients. Siriraj Med J 2014; 66: Gonzalez H, Minville V, Delanoue K, Mazerolles M, Concina D, Fourcade O. The importance of increased neck circumference to intubation difficulties in obese patients. Anesth Analg 2008; 106: Magalhaes E, Oliveira Marques F, Sousa Goveia C, Araujo Ladeira LC, Lagares J. Use of simple clinical predictors on preoperative diagnosis of difficult endotracheal intubation in obese patients. Braz J Anesthesiol 2013; 63: Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 39: Kim WH, Ahn HJ, Lee CJ, Shin BS, Ko JS, Choi SJ, et al. Neck circumference to thyromental distance ratio: a new predictor of difficult intubation in obese patients. Br J Anaesth 2011; 106: Arne J, Descoins P, Fusciardi J, Ingrand P, Ferrier B, Boudigues D, et al. Preoperative assessment for difficult intubation in general and ENT surgery: predictive value of a clinical multivariate risk index. Br J Anaesth 1998; 80: Ittichaikulthol W, Chanpradub S, Amnoundetchakorn S, Arayajarernwong N, Wongkum W. Modified Mallampati test and thyromental distance as a predictor of difficult laryngoscopy in Thai patients. J Med Assoc Thai 2010; 93: Kikkawa YS, Tsunoda K, Niimi S. Prediction and surgical management of difficult laryngoscopy. Laryngoscope 2004; 114: Hawkins JL. Providing anesthesia for the morbidly obese patient. Semin Anesth Perioper Med Pain 1995; 14: Shiga T, Wajima Z, Inoue T, Sakamoto A. Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anesthesiology 2005; 103: Bagley SC, White H, Golomb BA. Logistic regression in the medical literature: standards for use and reporting, with particular attention to one medical domain. J Clin Epidemiol 2001; 54: Deurenberg P, Deurenberg-Yap M, Guricci S. Asians are different from Caucasians and from each other in their body mass index/body fat per cent relationship. Obes Rev 2002; 3: J Med Assoc Thai Vol. 99 No

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